Knocking on Heaven's Door - Lisa Randall [182]
Of course, since we know neither the exact particle mass nor the precise interactions (not to mention the model of which this stable particle might be a part), we don’t yet know if the numbers work out exactly. But the fortuitous, albeit rough, agreement between numbers associated with what on the surface appear to be two entirely different phenomena is intriguing, and might well be a signal that weak-scale physics accounts for the dark matter in the universe.
This type of dark matter candidate has become generically known as a WIMP, or a Weakly Interacting Massive Particle. The word “weak” here is a descriptive term and not a reference to the weak force—a WIMP would interact even more weakly than the Standard Model’s weakly interacting neutrinos. Without more direct evidence for dark matter and its properties of the sort the LHC might reveal, we won’t know whether dark matter indeed consists of WIMPs. This is why we need experimental searches such as those we now consider.
DARK MATTER AT THE LHC
The intriguing possibility of producing dark matter is one reason cosmologists are curious about the physics of the weak energy scale and what the LHC might find. The LHC has just the right energy to search for a WIMP. If dark matter is indeed composed of a particle associated with the weak energy scale as the above calculation suggests, it just might be created at the LHC.
Even if that’s the case, however, the dark matter particle won’t necessarily be discovered. After all, dark matter doesn’t interact a lot. Due to their limited interactions with Standard Model matter, dark matter particles certainly won’t be produced directly or found in a detector. Even if produced, they will just fly through. Nonetheless, all is not lost (even if the dark matter particle will be). Any solution to the hierarchy problem will contain other particles—most of which have stronger interactions. Some of these might be copiously produced and subsequently decay into dark matter that will then carry away undetected momentum and energy.
Supersymmetric models are the most well-studied weak scale models of this type that naturally contain a viable dark matter candidate. If supersymmetry applies in the world, the lightest supersymmetric particle (LSP) might constitute the dark matter. This lightest particle, which carries zero electric charge, interacts too weakly to be produced on its own sufficiently often to find. However, gluinos—supersymmetric partners of the strong-force-communicating gluons, and squarks—supersymmetric partners of quarks—would be created if they exist and are in the right mass range. And, as was discussed in Chapter 17, both of these supersymmetric particles would eventually decay into the LSP. So even though a dark matter particle wouldn’t be produced directly, decays of other more prolifically created particles could conceivably create LSPs at an observable rate.
Other weak-scale dark matter scenarios that have testable consequences would have to be produced and “detected” in much the same way. The mass of the dark matter particle should be around the weak scale energy that the LHC will study. Those particles won’t be produced directly because of their feeble interaction strength, but many models contain other new particles that could decay into them. We might then learn of the dark matter particle’s existence and possibly its mass through the missing momentum it carries away.
Finding dark matter at the LHC would certainly be a major accomplishment. If it is found there, experimenters could even study some of its properties in detail. However, to really establish that a particle found at the LHC indeed constitutes the dark matter would require supplementary evidence. That is what detectors on the ground and in space might provide.
DIRECT DETECTION DARK MATTER EXPERIMENTS